The invention relates to a safety arrangement for power-assisted steerings having the features set forth herein. More specifically, the invention relates to a safety arrangement for power assisted hydraulic steering systems in which a further hydraulic system is supplied from the same pump and reservoir. The arrangement insures operation of the hydraulic power steering system even if a failure or leak occurs in the other hydraulic system.
A priority valve constructed to operate by pressure balance may be provided in known manner in a fluid line leading from the pump to the additional or auxiliary system whereas the steering valve for the power-assisted steering is connected directly to the pump. The pressure difference occurring at the steering valve acts on the control piston of the priority valve in such a manner that with increasing power requirement of the steering the influx of fluid to the hydraulic system is throttled.
Furthermore, a flow-dividing valve is known (German Patent Application No. P2723490) which is also constructed as pressure balance. The input of the flow-dividing valve is connected to the pump whereas one output is connected to the line leading to the steering valve and a second output is connected to the line leading to a hydraulic system. In a regulating position in which both steering valve and hydraulic system are supplied with fluid the branch leading to the steering valve is provided with a load pressure compensation so that even with variable load pressure of the power-assisted steering the fluid amount flowing to the steering valve is kept constant. From this regulating position the priority valve is switched to a switch position in which the connection to the hydraulic system is interrupted and the entire fluid amount delivered by the pump passes to the line leading to the power-assisted steering.
If, with these types of devices, a leak occurs in the area of the additional hydraulic system, the entire fluid supply can finally be lost because the priority valve is not able to detect the fault. To avoid failure of the steering, the power-assisted steering may be connected to a separate pump which is supplied with fluid either from a separate reservoir or from a suction connection of which is located in the common reservoir deeper than the suction connection of the pump supplying the hydraulic system with fluid. If the fluid level in the reservoir drops, the influx to the hydraulic system is interrupted. However, with such a safety arrangement a second pump is necessary and consequently the economical solution with one pump and priority valve cannot be adopted.
The problem underlying the invention thus resides in providing in the supply of a power-assisted steering and an additional hydraulic system via a pump a safety arrangement which makes it possible when the level in the reservoir drops to continue reliably to supply the power-assisted steering with fluid.